Solid-state disconnect device

ABSTRACT

A solid-state disconnect device capable of isolating and protecting circuits and equipment from overloads and undesired transients is presented. The protection device includes at least one depletion mode circuit block having three terminals (drain, gate, and source), which in its simplest form is implemented by a single n-channel depletion mode field-effect transistor, and two enhancement mode circuit blocks each having three terminals (drain, gate and source), each implemented in simplest form by a single n-channel enhancement mode field-effect transistor. The current conducting path of the first enhancement mode circuit block is connected in series with the current conducting path of the depletion mode circuit block. The drain terminal of the second enhancement mode circuit block is connected through a current limiting load to both the gate terminal of the second enhancement mode circuit block and the drain terminal of the first enhancement mode circuit block. The gate terminal of the first enhancement mode circuit block is connected to the drain terminal of the second enhancement mode circuit block. The source terminals of the two enhancement circuit blocks are both connected to the gate terminal of the depletion mode circuit block. Unidirectional and bidirectional embodiments are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

None.

FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

None.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to the field of protection devices. Inparticular, the invention is a solid-state disconnect device capable ofpreventing the flow of undesirable voltage or current transients and/orisolating equipment from undesirably high voltages or currents.

2. Background

Traditional protection devices which function as circuit disconnects,examples including fuses and electromechanical circuit breakers, areinherently problematic. For example, such devices have low-switchingspeeds, require replacement after each trip event, are prone to arcingand switching bounce with associated noise and wear problems, and/or areoften large on a volume basis resulting in an unwieldy package.

Solid-state technology applied to such protection devices avoids thesedisadvantages while offering higher reliability and longer lifetime.Accordingly, solid-state circuit disconnects have become desirablealternatives to traditional protection devices.

Various solid-state disconnect devices have been devised utilizingcomplex circuitry with an additional power source, examples includingdevices by Billings et al. in U.S. Pat. No. 4,245,184 entitled ACSolid-State Circuit Breaker, Witmer in U.S. Pat. No. 5,606,482 entitledSolid State Circuit Breaker, Partridge in U.S. Pat. No. 6,104,106entitled Solid State Circuit Breaker, and Covi et al. in U.S. Pat. No.6,515,840 entitled Solid State Circuit Breaker with Current OvershootProtection. However, it is more desirable that a circuit protectiondevice includes two terminals and no additional power source, much likea fuse.

Harris, in U.S. Pat. No. 5,742,463 entitled Protection Device usingField Effect Transistors, describes a two-terminal, solid-stateprotection device without an additional power supply. Severaldisadvantages are noteworthy. First, the protection device requiresp-channel, high-voltage field-effect-transistors (FETs) which generallyhave a high conduction resistance because of low hole mobility. Second,the protection device requires both gate-to-source terminals andgate-to-drain terminals of the FETs to have the same high-voltageblocking capability which is difficult to achieve because FETs block thevoltage between the drain-to-source and drain-to-gate terminals, and thegate-to-source terminal generally does not have such blockingcapability. For example, the gate and source terminals of all known FETstypically block a few volts to a few tens of volts because of alow-voltage Schottky barrier diode in MESFETs, a low-voltage PN diode inJFETs, and a low-voltage MOS capacitor in MOSFETs. Third, the protectiondevice requires a large number of FETs connected in series to increasethe voltage blocking capability of the device, inevitably resulting in ahigher conduction loss. Fourth, the tripping current is difficult tocontrol with the protection device.

Therefore, what is required is a solid-state disconnect device havingtwo terminals and no additional power supply that avoids thedisadvantages of the related arts.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a solid-statedisconnect device having two terminals and no additional power supplythat avoids the disadvantages of the related arts.

For high-voltage and high-current protection, silicon (Si) power devicesare generally connected in parallel with the system to be protectedbecause such devices have a high insertion loss and a large capacitancewhich dissipate too much power in a power system or reduce thecommunication bandwidth in a communication system when connecteddirectly in series. With the introduction of wide bandgap semiconductorpower devices, such as those based on silicon carbide (SiC), galliumnitride (GaN), and diamond, direct serial connectivity of protectiondevices in the circuit or system to be protected is more feasible atvoltages over 10,000 volts, because these wide bandgap switches have aspecific ON-state resistance nearly a thousand times lower than siliconcomponents. The benefits include improved insertion loss, speed, andbandwidth and lower costs.

The solid-state disconnect device described herein is a two-terminalprotection device connectable in series between a power supply and aload while avoiding an additional power source. The disconnect devicefurther includes a depletion mode circuit block (DCB), an enhancementmode circuit block (ECB), an enhancement mode circuit block (ECB) with apositive threshold voltage, and a current limiting load (CLL). The CLLcould be a simple resistor or a dynamic load formed by a circuit havinga resistance that increases with terminal voltage.

The DCB is broadly defined as a circuit block having three terminals,namely a drain, source, and gate, and a negative threshold voltage thatcontrols the current conduction between the drain and source terminals.When the voltage across the gate and source terminals is larger than thenegative threshold voltage, the circuit block is turned ON and currentconduction between the drain and source terminals increases with anincrease in voltage across the gate and source terminals. When thevoltage across the gate and source terminals is below the negativethreshold voltage, the circuit block is OFF and negligible current flowsthrough the circuit block.

The DCB could include a variety of designs composed of single andmultiple components. In its simplest form, the DCB could be a singledepletion mode n-channel transistor. In other embodiments, the DCB couldbe composed of any number of depletion mode n-channel transistorsconnected in a serial and/or a parallel arrangement. The depletion moden-channel transistors should withstand the surge voltage and surgecurrent of the application. Exemplary depletion mode components includejunction field effect transistors (JFETs), metal oxide semiconductorfield effect transistors (MOSFETs), static induction transistors, andinsulated-gate bipolar transistors (IGBTs); however, depletion modeJFETs are generally preferred. The desired features of the depletionmode transistors include a low insertion loss, a low capacitance, andability to withstand a surge current and voltage without breakdown.Depending on the specific application, the voltage blocking capabilitycould be up to a few tens of thousands of volts while the currentcapability could be up to a few thousands of amperes.

The ECB is broadly defined herein as a circuit block having threeterminals, namely a drain, source, and gate, and a positive thresholdvoltage. When the voltage across the gate and source terminals is largerthan the positive threshold voltage, the circuit block is turned ON andcurrent can flow between the drain and source terminals. When thevoltage across the gate and source terminals is less than the positivethreshold voltage, the circuit block is OFF.

The ECB could include a variety of designs composed of single andmultiple components. In its simplest form, the ECB could be a singleenhancement mode n-channel transistor. In other embodiments, any numberof enhancement mode n-channel transistors could be connected in a serialand/or parallel arrangement. Exemplary enhanced mode components includejunction field effect transistors (JFETs), metal oxide semiconductorfield effect transistors (MOSFETs), static induction transistors, andinsulated-gate bipolar transistors (IGBTs); however, enhancement moden-channel MOSFETs are generally preferred.

In its simplest form, the solid-state disconnect or protection devicecould include a DCB, a first ECB, a second ECB, and a CLL. The DCB andfirst ECB are connected in a serial arrangement so as to form thecurrent path of the two-terminal protection device. The source terminalof the DCB is connected to the drain terminal of the first ECB. Thedrain terminal of the second ECB is connected to the gate terminal ofthe first ECB and further connected through the CCL to the drainterminal of the first ECB. The gate terminal of the DCB and the sourceterminal of the second ECB are connected and further connected to thesource terminal of the first ECB. The gate terminal of the second ECB isconnected to the drain terminal of the first ECB. The voltage dropacross the protection device, which increases as surge currentincreases, is used to control the OFF and ON status of the second ECB.The OFF and ON status of the second ECB in turn controls the OFF and ONstatus of the first ECB which in turn further controls the OFF and ONstatus of the DCB. The described circuit architecture providesprotection functionality to a disconnect system or equipment protectedfrom a high-voltage spike, and undesirable surge current flowing fromthe drain-to-source of the DCB and further through the drain-to-sourceof the first ECB.

The protection device described herein could include optional componentsto further improve the performance of the disconnect system. Forexample, in applications requiring current to flow in both directions, acurrent bypass component, which conducts current in one direction andblocks current in the other direction, one non-limiting example being aSchottky diode, could be connected to the source and drain terminals ofthe DCB or the first ECB to allow current to flow from source to drainterminals, when the DCB or the first ECB has poor or no currentconduction capability from the source-to-drain terminals. In anotherexample, a low-pass RC network could be connected between the drainterminal of the DCB and the source terminal of the first ECB to filterout high-frequency voltage spikes generated during transient andtripping events. In yet another example, a voltage-limiting component,examples including but not limited to a reverse selenium rectifier, avaristor, a simple resistor, a circuit block, or preferably avoltage-clamping Zener diode, could be connected to the source and gateterminals of each of the ECBs to prevent the gate from electricbreakdown because of a high voltage. In still another example, atemperature compensation component, one non-limiting example being athermistor with a negative temperature coefficient in resistance, couldbe connected between the gate and source terminals of the second ECB tomake the tripping current insensitive to temperature change. In stillyet another example, a variable resistor could be connected between thegate and source terminals of the second ECB to adjust the trippingcurrent. In yet still another example, a capacitor can be connectedbetween the gate and source terminals of the second ECB to suppressvoltage spikes across the gate and source terminals of the second ECB soas to prevent premature trigger of the device. The gate terminal of thesecond ECB should be connected through a current limiting load insteadof connected directly to the drain terminal of the first ECB, when atleast one of the aforementioned optional components is connected betweenthe gate and source terminals of the second ECB in order to ensure thata large enough voltage is established across the first ECB when thefirst ECB is turned OFF so as to enable the turn OFF of the DCB.

For higher current protection, it is preferable that the drain terminalof the second ECB be connected through a CLL to the drain of the DCB,rather than to the drain of the first ECB, and the gate of the secondECB be connected through another CLL to the drain of the DCB, ratherthan directly to the drain of the first ECB.

The protection device described herein could be arranged in either aunidirectional or a bidirectional circuit. For unidirectionalprotection, the device could include one form of the circuit describedabove. For bidirectional protection, two such circuits could beconnected in mirror symmetry in a serial arrangement with a load. In thelatter, it is preferred for the drain terminals of the DCBs of the twocircuits to be connected, although the two source terminals of the DCBscould also be connected in mirror symmetry. For bidirectionalprotection, it is further preferred that both gates of the ECBs in onecircuit be connected through their respective CLLs to the sourceterminal of the first ECB in the other circuit instead of to the drainof the DCB in the same circuit, although connecting only one of the twogates of the ECBs in one circuit to the source of the first ECB in theother circuit is also possible. Optional components described hereincould be added to bidirectional embodiments of the invention asrequired.

Several advantages are offered by the invention. The disconnect deviceprovides unidirectional and bidirectional protection against surgecurrent and voltage, resets automatically, has a very-high trippingspeed on the order of microseconds to sub-microseconds, is capable ofprotecting both DC and AC power systems, facilitates adjustments to andcontrol of tripping current, and is insensitive to temperaturevariations.

REFERENCE NUMERALS

-   1 Terminal-   2 Terminal-   3 Terminal-   4 Terminal-   5, 5 a, 5 b DCB-   6, 6 a, 6 b First ECB-   7, 7 a, 7 b Second ECB-   8, 8 a, 8 b CLL-   9, 9 a, 9 b Second CLL-   10, 10 a, 10 b Voltage limiting component-   11, 11 a, 11 b Voltage limiting component-   12, 12 a, 12 b Temperature compensation component-   13, 13 a, 13 b Variable resistor-   14, 14 a, 14 b Capacitor-   15, 15 a, 15 b Resistor-   16, 16 a, 16 b Capacitor-   17, 17 a, 17 b Current bypass component-   18, 18 a, 18 b Current bypass component-   19 Capacitor-   20 Resistor-   30, 30 a, 30 b First node-   31, 31 a, 31 b Second node-   32, 32 a, 32 b Third node-   33, 33 a, 33 b Fourth node-   34, 34 a, 34 b Fifth node-   35, 35 a, 35 b Sixth node-   36, 36 a, 36 b Seventh node-   40 Depletion mode FET-   41 Feedback resistor-   42 Depletion mode FET-   43 Resistor-   44 Depletion mode FET-   50 Protection device-   52 Protection device-   54 Protection device-   56, 56 a, 56 b Protection device-   58 Protection device-   60 Protection device-   61 Protection device-   62 Bidirectional protection device-   63 Bidirectional protection device-   64 Bidirectional protection device-   65 Bidirectional protection device-   81 Terminal-   82 Terminal-   83 Terminal-   84 Terminal

BRIEF DESCRIPTION OF THE INVENTION

Additional aspects, features, and advantages of the invention will beunderstood and will become more readily apparent when the invention isconsidered in the light of the following description made in conjunctionwith the accompanying drawings.

FIG. 1 a is a circuit diagram illustrating a solid-state protectiondevice in accordance with an embodiment of the invention.

FIG. 1 b is a circuit diagram illustrating the solid-state protectiondevice from FIG. 1 a with optional components in accordance with anembodiment of the invention.

FIG. 1 c is a circuit diagram illustrating the solid-state protectiondevice from FIG. 1 b wherein the depletion mode circuit block is adepletion mode n-channel JFET and the enhancement mode circuit blocksare each an enhancement mode MOSFET in accordance with an embodiment ofthe invention.

FIG. 2 a is a circuit diagram illustrating alternate connectivity of theCLLs in the solid-state protection device from FIG. 1 b in accordancewith an embodiment of the invention.

FIG. 2 b is a circuit diagram illustrating alternate connectivity of theCLLs in the solid-state protection device from FIG. 1 b in accordancewith an embodiment of the invention.

FIG. 2 c is a circuit diagram illustrating alternate connectivity of theCLLs in the solid-state protection device from FIG. 1 b in accordancewith an embodiment of the invention.

FIG. 2 d is a circuit diagram illustrating an alternate configuration ofthe solid-state protection device in FIG. 2 a wherein the depletion modecircuit block is a depletion mode n-channel JFET and the enhancementmode circuit blocks are each an enhancement mode MOSFET in accordancewith an embodiment of the invention.

FIG. 3 a is a circuit diagram illustrating a bidirectional protectiondevice including a pair of solid-state protection devices from FIG. 2 aconnected in a serial arrangement in accordance with an embodiment ofthe invention.

FIG. 3 b is a circuit diagram illustrating a bidirectional protectiondevice including a pair of solid-state protection devices from FIG. 2 dconnected in a serial arrangement in accordance with an embodiment ofthe invention.

FIG. 4 a is a circuit diagram illustrating an alternate configuration ofthe bidirectional protection device from FIG. 3 a in accordance with anembodiment of the invention.

FIG. 4 b is a circuit diagram illustrating an alternate configuration ofthe bidirectional protection device from FIG. 3 b in accordance with anembodiment of the invention.

FIG. 5 a is a plot illustrating the simulated current waveform inexpanded scale produced by the bidirectional protection device in FIG. 4b protecting a single-phase 10-kilowatt DC-to-DC converter systempowered by a 200 volt battery when a fault causes the battery dischargecurrent to rise sharply.

FIG. 5 b is a plot illustrating the simulated current waveform producedby the bidirectional protection device in FIG. 4 b protecting asingle-phase 10-kilowatt DC-to-DC converter system powered by a 200 voltbattery when a fault causes the battery discharge current to risesharply and after the system resumes normal operation after the fault iscleared.

FIG. 6 is a plot illustrating the simulated current waveform in expandedscale produced by the bidirectional protection device in FIG. 4 bprotecting a single-phase 10-kilowatt DC-to-DC converter system poweredby a 200 volt battery when a fault causes the battery charge current torise sharply.

FIG. 7 is a plot illustrating the effects of temperature compensationcomponents on the trip current of the bidirectional protection device inFIG. 4 b.

FIG. 8 is a plot illustrating the effects of current bypass components(power diodes) on reducing the forward voltage drop across thebidirectional protection device in FIG. 4 b.

FIG. 9 is a plot illustrating the lower conduction voltage drop of thebidirectional protection device in FIG. 4 b in comparison to the forwardvoltage drop of the bidirectional protection device in FIG. 3 b.

FIG. 10 a is a plot illustrating the measured current tripping waveformsof the bidirectional protection device in FIG. 4 b with a trip currentof 53 amperes.

FIG. 10 b is a plot illustrating the measured current tripping waveformsof the bidirectional protection device in FIG. 4 b with a trip currentof 55 amperes.

DETAILED DESCRIPTION

Reference will now be made in detail to several preferred embodiments ofthe invention that are illustrated in the accompanying drawings.Wherever possible, same or similar reference numerals are used in thedrawings and the description to refer to the same or like parts. Nodesare referenced for descriptive purposes only and do not necessarilyrepresent a structure or element of the invention. The drain, gate, andsource terminals of components are identified by the letters D, S, andG, respectively, in FIGS. 1 a-1 c, 2 a-2 d, 3 a, 3 b, 4 a, and 4 b.

Referring now to FIG. 1 a, an embodiment of the protection device 50 isshown for a unidirectional device capable of preventing a surge currentthat exceeds a preset trip value between terminals 1 and 3, bydisconnecting a load at terminals 3 and 4 in response to a voltagesurge. The protection device 50 is connected in series between a sourceor voltage supply across terminals 1 and 2 with the polarity shown andthe load across terminals 3 and 4. The protection device 50 includes aDCB 5, a first ECB 6, a second ECB 7, and a CLL 8 connected as shown vianodes 30-36. The DCB 5 is a depletion mode circuit block with a negativethreshold voltage. The first ECB 6 and second ECB 7 are enhancement modecircuit blocks with a positive threshold voltage. The drain terminal ofthe DCB 5 is connected to the first node 30. The gate terminal of theDCB 5 and the source terminals of the first ECB 6 and the second ECB 7are connected to the third node 32. The source terminal of the DCB 5 andthe drain terminal of the first ECB 6 are connected to the second node31. The gate terminal of the first ECB 6 and the drain terminal of thesecond ECB 7 are connected to the fifth node 34. The gate terminal ofthe second ECB 7 is connected to the sixth node 35. The CLL 8 isconnected between the fourth node 33 and fifth node 34. Thereafter, thesecond node 31, fourth node 33, sixth node 35, and seventh node 36 arearranged and connected as shown.

The functionality of the protection circuit 50 is described with furtherreference to FIG. 1 a. In normal operation, a positive current flowsfrom terminal 1 to terminal 3. The voltage drop across the second node31 and the third node 32 is larger than the threshold voltage of thefirst ECB 6 but smaller than the threshold voltage of the second ECB 7.As such, the first ECB 6 is maintained in the ON-state while the secondECB 7 is in the OFF-state during normal operating conditions. The DCB 5is in the ON-state as a normally ON device and the magnitude of itsgate-to-source voltage, approximately equal to the magnitude of thevoltage drop across the third node 32 and second node 31, is smallerthan the magnitude of its negative threshold voltage.

When a surge current or sudden voltage increase at terminal 1 attemptsto cross to terminal 3, the voltage drop between the second node 31 andthird node 32 momentarily increases, resulting in a momentary increasein the gate voltage of the second ECB 7. The second ECB 7 remains in theOFF-state until the voltage drop between the second node 31 and thirdnode 32 reaches the threshold voltage of the second ECB 7 placing thesecond ECB 7 in the ON-state at a predetermined surge current. The peaksurge current at which the disconnect trips is therefore controlled bythe magnitude of the threshold voltage of the second ECB 7 and theforward characteristics of the first ECB 6. After the second ECB 7 isturned ON, the voltage drop across the drain terminal, at the fifth node34, and the source terminal, at the third node 32, of the second ECB 7is decreased to a value less than the threshold voltage of the first ECB6 so as to turn OFF the first ECB 6. Once the first ECB 6 is OFF,current is forced to pass through the CLL 8 and the second ECB 7. TheCLL 8 produces a voltage drop across the second node 31 and third node32 coupled to the source and gate terminals of the DCB 5, respectively.The first ECB 6 should be sufficiently capable of handling adrain-to-source voltage larger than the numerical value of the negativethreshold voltage of the DCB 5. A typical threshold voltage for the DCB5 could be in the range of −1 volts to −50 volts, although other valuesare possible. When the drain-to-source voltage of the first ECB 6increases to a value greater than the numerical value of the thresholdvoltage of the DCB 5, the DCB 5 is turned OFF. Thereafter, theprotection circuit 50 enters the OFF-state so as to prevent the surgecurrent from reaching and damaging the load and to disconnect the loadfrom the high-voltage spike. The protection device 50 resumes normaloperation automatically once the voltage drop across the device fromterminals 1 to 3 decreases to a value causing the second ECB 7 to turnOFF after the fault causing the current surge is cleared.

A single DCB 5 comprising a high-voltage depletion mode n-channeltransistor could be employed to disconnect the surge voltage because thevoltage blocked by the DCB 5 is across the drain and source rather thanacross the gate and source as provided in U.S. Pat. No. 5,742,463. Forexample, a single silicon carbide FET could be used to block a voltagesurge over 10,000 volts, non-limiting examples being a 10 kV, 5 A 4H—SiCPower DMOSFET and a 10 kV, 87 mΩ-cm² normally-OFF 4H—SiC VerticalJunction Field-Effect Transistor. The corresponding DCB 5 and first ECB6 should have similar current handling capability dependent on thespecific application, which could range from a few milli-amperes to afew tens of thousands of amperes. The second ECB 7 would not necessarilyrequire a high current capability because it is limited by the CLL 8.

Referring now to FIG. 1 b, the protection device 50 from FIG. 1 a isshown with a variety of optional components to form various alternateprotection devices 52. The architecture of the protection device 52 isidentical to that in FIG. 1 a, except where otherwise indicated. Forexample, a second CLL 9 could be provided between the sixth node 35 andseventh node 36. A voltage limiting component 10 could be connected asneeded between the gate and source terminals of the first ECB 6 toprevent electric breakdown of the gate due to a high voltage event.Another voltage limiting component 11 could be connected as neededbetween the gate and source terminals of the second ECB 7, so as toprevent electric breakdown of the gate due to a high voltage event.Although Zener diodes are represented in FIG. 1 b, any voltage-limitingcomponents, non-limiting examples including reverse selenium rectifiers,varistors made from various materials, a simple resistor, or a circuitblock, but preferably a voltage-clamping Zener diode, could be employedas one or both voltage limiting components 10, 11.

Referring again to FIG. 1 a, the ON-state resistances of the DCB 5 andfirst ECB 6 increase and the threshold voltage of the second ECB 7decreases as the temperature of the protection device 50 increases. As aconsequence, the trip current of protection device 50 will decrease.

Referring again to FIG. 1 b, the protection device 52 could include atemperature compensation component 12 with a negative temperaturecoefficient (NTC) in resistance, a non-limiting example being an NTCthermistor, in order to maintain a relatively constant trip current. Thetemperature compensation component 12 could be connected between thegate and source terminals of the second ECB 7. The temperaturecompensation component 12 and second CLL 9 form a voltage divider. Adecrease in the resistance of the temperature compensation component 12with an increase in temperature tends to reduce the voltage drop acrossthe gate and source terminals of the second ECB 7. As a result, anincrease in the ON-resistance of the DCB 5 and the first ECB 6 and adecrease in the threshold voltage of the second ECB 7 due to atemperature increase are compensated by the decrease in the bias voltageacross the gate-to-source terminals of the second ECB 7. Hence, the tripcurrent of the protection device 52 could be nearly temperatureindependent over a specified range of temperatures.

Referring again to FIG. 1 b, an optional variable resistor 13 could alsobe connected between the gate and source terminals of second ECB 7 toadjust the voltage drop across the gate and source terminals so as toadjust the trip current. An optional capacitor 14 could be furtherconnected between the gate and source terminals of the second ECB 7 tosuppress potential voltage spikes across the gate and source terminalsto prevent a premature trigger of the second ECB 7. An optional RCnetwork could be further connected between the first node 30 and thethird node 32, preferably after the capacitor 14, to filter outhigh-frequency voltage spikes generated during transient and tripconditions. One non-limiting exemplary embodiment of the RC network is aresistor 15 and capacitor 16, as represented in FIG. 1 b.

Referring again to FIG. 1 b, the protection device 52 could furtherinclude current bypass components 17, 18 as required, which conductcurrent in one direction and block current in the other direction. Thesimplest form of a current bypass component is a Schottky diode,although other components are possible. The current bypass components17, 18 could be connected in parallel to the current conducting channelbetween the source and drain terminals of the DCB 5 and first ECB 6,respectively, when an application requires current to flow from terminal3 to terminal 1, and when the DCB 5 or first ECB 6 has poor or nocurrent conducting capability from source-to-drain terminals. Forexample, MOSFETs contain a built-in body diode between its source anddrain terminals which allows current conduction from source-to-drain butwith a relatively large voltage drop. As such, it is preferred that alow-voltage-drop current bypass diode be connected between the sourceand drain terminals of each MOSFET. In another example, IGBTs do notinclude a body diode so a low-voltage-drop current bypass diode could beincluded to provide reverse current conduction when a specificapplication requires current to flow from terminal 3 to terminal 1.

The functionality of the protection device 52 of FIG. 1 b is similar tothe protection device 50 of FIG. 1 a. The protection device 52 is aunidirectional protection device capable of preventing a surge currentthat exceeds a preset trip current to conduct from terminal 1 toterminal 3 and disconnecting the load from a voltage surge. However,current conduction in the protection device 52 is bidirectional.

The CLLs 8, 9 in FIGS. 1 a and 1 b could include a variety of devices.Each CLL 8, 9 could be a resistor with a predetermined value ofresistance. Preferably, each CLL 8, 9 could be a dynamic load formed bya circuit block having increased resistance with increased terminalvoltage in order to reduce power dissipation. For example, the CLL 8 inFIG. 1 a is shown including a depletion mode FET 40 and a feedbackresistor 41 capable of providing a small load resistance for a short RCcharge time for the first ECB 6 at low voltage and a very large loadresistance to limit the leakage current in a high-voltage blocking mode.

The DCBs 5, 5 a, 5 b described herein could include a variety of singleand multi-element devices. In one example, the DCBs 5, 5 a, 5 b could becomposed of any number of depletion mode n-channel transistors connectedin a serial and/or parallel arrangement. In another example, the DCB 5,5 a, 5 b could be a single depletion mode n-channel transistor,non-limiting examples including a depletion mode n-channel junctionfield effect transistor (JFET), a depletion mode n-channel metal oxidesemiconductor field effect transistor (MOSFET), and a depletion modeinsulated-gate bipolar transistor (IGBT).

The ECBs 6, 6 a, 6 b, 7, 7 a, 7 b described herein could include avariety of single and multi-element devices. In one example, the ECBs 6,6 a, 6 b, 7, 7 a, 7 b could include any number of enhancement moden-channel transistors connected in a serial or parallel arrangement. AnECB 6, 6 a, 6 b, 7, 7 a, 7 b in its simplest form could be a singleenhancement mode n-channel transistor, non-limiting examples includingan enhancement mode n-channel junction field effect transistor (JFET),an enhancement mode n-channel metal oxide semiconductor field effecttransistor (MOSFET), and an enhancement mode insulated-gate bipolartransistor (IGBT).

Referring now to FIG. 1 c, the protection device 54 is shown whereby theDCB 5 is a depletion mode n-channel JFET and the ECBs 6, 7 are each oneenhancement mode MOSFET. Components and architecture are identical tothose in FIG. 1 b, except where otherwise noted. For example, theoptional current bypass component 17 is not required because thedepletion mode n-channel JFET has good current conduction performancefrom source-to-drain terminals. The functionality of the protectiondevice 54 in FIG. 1 c is similar to the protection device 52 in FIG. 1b, in that the protection device 54 is a unidirectional device capableof preventing a surge current that exceeds a preset trip current tocross from terminal 1 to terminal 3 and disconnecting the load from avoltage surge.

The ECBs 6, 6 a, 6 b, 7, 7 a, 7 b described herein generally have agate-to-source voltage larger than the threshold voltage required toturn ON the circuit block. The higher the gate-to-source voltage is, thelower the ON-state voltage drop for the same current level is. For thefirst ECB 6 in FIG. 1 b, the ON-state voltage drop is fed back to itsgate to keep the ECB 6 in its ON-state. In general applications, it isdesired that the ON-state voltage drop of the first ECB 6 to be as lowas possible so as to reduce power loss. Therefore, the first ECB 6should have a positive but low threshold voltage.

FIGS. 2 a-2 d describe several alternate embodiments of the protectiondevice 52 in FIG. 1 b. Components and architecture are identical tothose in FIG. 1 b, except where otherwise noted. The protection devices56, 58, 60, and 61 allow bidirectional current conduction, although theprotection function is unidirectional. Operation of the protectiondevices 56, 58, 60, and 61 are similar to that of the protection device52 of FIG. 1 b. The circuit in FIG. 2 a is a preferred embodiment.

Referring now to FIG. 2 a, an improved embodiment the protection device56 is shown which reduces the ON-state voltage drop of the first ECB 6in FIG. 1 b. The fourth node 33 and seventh node 36 are now connected tothe first node 30 rather than to second node 31. As such, the entirevoltage drop across the protection device 56, including the voltage dropacross the drain and source terminals of the DCB 5 and across the drainand source terminals of the first ECB 6, is employed to bias the gateand source terminals of the first ECB 6. For the same level of currentconducting through the protection device 56, the voltage available tobias the gate and source terminals of the first ECB 6 in FIG. 2 a ismuch larger than that in FIG. 1 b. Therefore, the forward voltage dropof the first ECB 6 in FIG. 2 a is smaller than the forward voltage dropof the first ECB 6 in FIG. 1 b. Accordingly, the ON-state forwardvoltage drop or the insertion loss of the protection device 56 issmaller than that of the previously described protection device 52.

Referring again to FIG. 2 a, the voltage drop across the gate and sourceterminals of the second ECB 7 is much larger than that in FIG. 1 b forthe same current level because of the connection of the seventh node 36to first node 30. This allows the threshold voltage of the second ECB 7in FIG. 2 a to be larger than that in FIG. 1 b. A larger thresholdvoltage in practice is easier to achieve.

Referring again to FIG. 2 a, the voltage limiting component 11 isrequired to prevent the gate-source terminals of the second ECB 7 frombeing exposed to a damaging high voltage, and second CLL 9 is requiredto support most of the voltage drop across the protection device 56 andto limit the current flowing through the voltage limiting component 11after the protection device 56 is tripped into the blocking OFF-state.The CLL 8 will also support most of the voltage drop across protectiondevice 56 and limit the current flowing through the second ECB 7 and thevoltage limiting component 10 after the protection device 56 is trippedinto the blocking OFF-state. A resistor with a predetermined value ofresistance could be employed as the CLL 8 and second CLL 9; however, adynamic load formed by a circuit block is otherwise preferred, asdescribed herein. The CLL 8 and second CLL 9 are preferred to be adepletion-mode transistor and a feedback resistor as shown in FIG. 1 a,except in this embodiment the depletion-mode transistor should have avoltage blocking capability similar to that of the DCB 5, because theCLLs 8, 9 could be subject to a high surge voltage. The transistors ofthe CLL 8 and second CLL 9 do not require high current capabilitybecause high current is not gene rally conducted through these elements.

The operation of the protection device 56 in FIG. 2 a is similar to theprotection device 52 in FIG. 1 b. As illustrated in FIG. 2 a, the sourceor supply voltage is connected across terminals 1 and 2 with thepolarity shown and the load is connected across terminals 3 and 4. Innormal operation, both the DCB 5 and first ECB 6 are ON, and the secondECB 7 is OFF. When a surge current enters from terminal 1 to terminal 3,the voltage drop across protection device 56 will momentarily increase,resulting in a momentary increase in the voltage drop across the gateand source terminals of the second ECB 7. Thereafter, the second ECB 7turns ON when the voltage drop across its gate and source terminalsreaches the threshold voltage of the second ECB 7. After the second ECB7 is turned ON, the voltage drop across its drain and source terminalsdecreases until it is lower than the threshold voltage of the first ECB6 so that the first ECB 6 is turned OFF. Once the first ECB 6 is OFF,the voltage drop across the drain and source terminals of the first ECB6 increases substantially which in turn turns OFF the DCB 5 to block thesurge voltage, which could be up to thousands or tens of thousands ofvolts. The result is that the protection device 56 effectively isolatesthe load from the supply voltage and any damaging current and voltage.

Referring now to FIG. 2 b, the fourth node 33 is now connected to thesecond node 31 and the seventh node 36 is connected to the first node30. The protection device 58 does not necessarily have a betterinsertion loss than the device in FIG. 1 b, but rather allows thethreshold voltage of the second ECB 7 to be much larger than in FIG. 1b.

Referring now to FIG. 2 c, the seventh node 36 is now connected to thesecond node 31 and the fourth node 33 is connected to the first node 30.The protection device 60 improves the insertion loss otherwiseachievable by the protection device 52 in FIG. 1 b.

Referring now to FIG. 2 d, the protection device 56 in FIG. 2 a is shownwith a depletion mode n-channel JFET at the DCB 5 and an enhancementmode MOSFET at the first ECB 6 and second ECB 7. The current bypasscomponent 17 in FIG. 2 a is not required because the depletion moden-channel JFET has good current conduction capability fromsource-to-drain terminals.

Bidirectional power systems, one non-limiting example being abidirectional DC-DC converter, are gaining increased attentions in awide range of applications including hybrid and electric vehicles wherea battery delivers and receives energy. Bidirectional power systemsrequire bidirectional protection devices. In accordance with embodimentsof the invention, a bidirectional protection device could be constructedwith two unidirectional protection devices.

Referring now to FIG. 3 a, a bidirectional protection device 62 is shownconstructed with two protection devices 56 a, 56 b, as described in FIG.2 a. A bidirectional source or supply voltage is connected acrossterminals 81 and 82 and a load is connected across terminals 83 and 84.The bidirectional protection device 62 is capable of protecting bothload and source from excessive positive and negative current and voltagesurges.

The protection device 56 b is identical in its construction to theprotection device 56 in FIG. 2 a and similar in operation thereto inthat it is operative to limit the positive surge current conducting fromterminal 81 to terminal 83. The protection device 56 b includes notationsimilar to that in FIG. 2 a to identify the various components andnodes, except that the reference numerals for are distinguished by thesuffix “b”.

The protection device 56 a is identical in its construction to theprotection device 56 in FIG. 2 a and operates in a similar manner to theprotection device 56 b, except that it is responsive to limit thenegative surge of current from terminal 83 to terminal 81. Theprotection device 56 a includes notation similar to that in FIG. 2 a toidentify the various components and nodes, except that the referencenumerals for are distinguished by the suffix “a”.

Referring again to FIG. 3 a, the relative positions of the protectiondevices 56 a and 56 b could be transposed, meaning one protection device56 a is closer to the load than the other protection device 56 b, thethird node 32 a is connected to the other third node 32 b, the firstnode 30 a is connected to one terminal 83 instead of to the first node30 b, and the first node 30 b is connected to the other terminal 81.However, it is preferred that the first node 30 a and other first node30 b be connected together as illustrated in FIG. 3 a, because the ONresistance of the bidirectional protection device 62 could be improved,as discussed herein.

Referring now to FIG. 3 b, the bidirectional protection device 63includes the bidirectional protection device 62 in FIG. 3 a wherein theDCBs 5 a, 5 b are each a depletion mode n-channel JFET and the ECBs 8 a,8 b, 9 a, 9 b are each an enhancement mode MOSFET. Components andarchitecture are otherwise identical to the bidirectional protectiondevice 62, except where otherwise noted. For example, the current bypasscomponents 17 a and 17 b in the bidirectional protection device 62 arenot required because of the conduction properties of the depletion moden-channel JFET from source to drain terminals. Operation of thebidirectional protection device 63 is similar to that of the device inFIG. 3 a.

The ON-resistance or insertion loss of the bidirectional protectiondevice 62 in FIG. 3 a could be reduced where the total voltage dropacross the protection devices 56 a, 56 b is used to bias the gates ofthe first ECBs 6 a, 6 b.

FIGS. 4 a and 4 b show two possible embodiments of a bidirectionalprotection device 64 and 65, respectively, that separately prevents asurge current that exceeds a preset tripping current from passingthrough the device and disconnects a load in response to a voltagesurge.

Referring now to FIG. 4 a, the bidirectional protection device 64 isshown based on the protection device 62 in FIG. 3 a. Components, nodes,and architecture are identical to that in FIG. 3 a, except whereotherwise noted. For example, the two optional RC networks of FIG. 3 a,each including a resistor 15 a or 15 b and a capacitor 16 a or 16 b, arecombined into one optional RC network in FIG. 4 a, formed by a capacitor19 and a resistor 20 connected parallel to the third nodes 32 a, 32 b.The fourth node 33 b and seventh node 36 b are connected to the thirdnode 32 a rather than to the first node 30 b in FIG. 3 a. The fourthnode 33 a and seventh node 36 a are connected to the third node 32 brather than to the first node 30 a in FIG. 3 a. In other embodiments, itis possible for only the fourth node 33 b or seventh node 36 b to beconnected to the third node 32 a and/or for only the fourth node 33 a orseventh node 36 a to be connected to the third node 32 b.

Referring again to FIG. 4 a, the CLLs 8 a, 8 b, 9 a, and 9 b shouldallow for bidirectional current limiting and high voltage handling. Insome embodiments, the CLLs 8 a, 8 b, 9 a, 9 b could each be implementedas a simple current limiting resistor. However, it is preferred thateach CLL 8 a, 8 b, 9 a, 9 b include a pair of depletion mode FETs 42, 44and a shared resistor 43, as shown in FIG. 4 a. This arrangement ensuresthat the resistance of each CLL 8 a, 8 b, 9 a, 9 b is low when thevoltage across each CLL 8 a, 8 b, 9 a, 9 b is low so as to achieve asmall RC charging time and a high resistance when the voltage acrosseach CLL 8 a, 8 b, 9 a, 9 b is high so as to limit leakage current whenthe bidirectional protection device 64 is in the blocking or OFF state.While JFETs are illustrated in FIG. 4 a, any depletion mode transistor,non-limiting examples including MOSFETs and IGBTs, are applicable to theCLLs 8 a, 8 b, 9 a, 9 b.

Referring again to FIG. 4 a, the entire voltage drop across thebidirectional protection device 64 is applied to the gate of the firstECB 6 b to reduce its ON resistance when current flows from one terminal81 to another terminal 83. Similarly, when current flows from oneterminal 83 to another terminal 81, the entire voltage drop across thebidirectional protection device 64 is applied to the gate of first ECB 6a to reduce its ON resistance. The increased gate-to-source bias voltagelowers the conduction power loss by reducing the conduction voltage dropof the first ECBs 6 a, 6 b.

Referring again to FIG. 4 a, the bidirectional protection device 64 isfunctionally similar to the bidirectional protection device 62 in FIG. 3a. When current flows from one terminal 83 to another terminal 81 undernormal operating conditions, the DCB 5 a and first ECB 6 a are in theON-state and the DCB 5 b and first ECB 6 b are in a reverse conductionstate. Conversely, when current flows from one terminal 81 to anotherterminal 83, the DCB 5 b and first ECB 6 b are in the ON-state and theDCB 5 a and first ECB 6 a are in the reverse conduction state. If any ofthe DCBs 5 a, 5 b and/or first ECBs 6 a, 6 b has either poor or noreverse current conduction capability, the reverse current flows throughthe appropriate current bypass component 17 a, 17 b, 18 a, 18 b.

If surge current flows from terminal 81 to terminal 83, then the voltagedrop across the bidirectional protection device 64, between nodes 32 aand 32 b, will momentarily increase, resulting in an increase in thegate-to-source bias voltage at the second ECB 7 b. The second ECB 7 b isturned ON at a predetermined surge current, dependent on the thresholdvoltage of the component. Once the second ECB 7 b is turned ON, thevoltage across the drain and source terminals of the second ECB 7 bdecreases to a very small value below the threshold voltage of the firstECB 6 b so that the first ECB 6 b is turned OFF. The shut-off of thefirst ECB 6 b causes a large voltage drop across the drain and source ofthe first ECB 6 b, which is provided as a reverse bias to thegate-to-source terminals of the DCB 5 b to choke off its conductingchannel and turn OFF the DCB 5 b. As a result, the bidirectionalprotection device 64 then disconnects the load from the surge currentand voltage. The surge voltage is mainly supported by the high voltageDCB 5 b, which, depending on the application, could be a single SiC FETcapable of blocking up to and over 10,000 volts and conductingmilli-amperes to thousands of amperes. Similarly, if a surge currentflows from terminal 83 to terminal 81, the second ECB 7 a is turned ON,depending on the threshold voltage the component, at a predeterminedsurge current causing the first ECB 6 a and DCB 5 a to turn OFF so thatthe bidirectional protection device 64 disconnects the load from thesurge current and voltage, and the high voltage drop across thebidirectional protection device 64 is mainly supported by thehigh-voltage DCB 5 a which, depending on applications, could also be asingle SiC FET sufficiently capable of blocking over 10,000 volts andconducting milli-amperes to thousands of amperes. The bidirectionalprotection device 64 resumes normal operation automatically once thevoltage drop across the device decreases to a value that turns OFFeither of the second ECBs 7 a, 7 b after the fault causing current surgeis cleared.

Referring now to FIG. 4 b, a bidirectional protection device 65 is shownbased on the bidirectional protection device 64 from FIG. 4 a, whereinthe DCBs 5 a, 5 b each include one depletion mode n-channel JFET and thefirst and second ECBs 6 a, 6 b, 7 a, 7 b each include one enhancementmode n-channel MOSFET. Components and architecture are otherwiseidentical to the bidirectional protection device 64 in FIG. 4 a, exceptwhere otherwise noted. The optional current bypass components 17 a, 17 bare not required because of the current conduction capability of thedepletion mode n-channel JFET. The operation of the bidirectionalprotection device 65 is similar to the bidirectional protection device64 in FIG. 4 a.

With reference to FIGS. 5 a, 5 b, 6, 7, 8, and 9, the bidirectionalprotection system 65 in FIG. 4 b was simulated with the PSpice® computerprogram, sold by Cadance Design Systems, Inc. of San Jose, Calif. Forsimplicity, the optional RC network including the capacitor 19 andresistor 20, the optional capacitors 14 a, 14 b, and the optionalvariable resistors 13 a, 13 b were not included in the simulations.Performance plots are for illustrative purposes only and are notintended to limit or otherwise restrict the scope of the embodimentsdescribed herein and their performance.

Referring now to FIGS. 5 a and 5 b, current versus time plots are shownfor the bidirectional protection device 65 connected in series between a200 volt battery and a single-phase 10 kilowatt DC-to-DC convertersystem powered by the 200 volt battery when a fault, such as a shortcircuit of the power transistor in the DC-DC converter, causes thebattery discharge current to rise sharply. The current is tripped at apre-designed level of 156 amperes and drops to below 2 amperes within 2microseconds and then quickly drops to near zero, as shown in FIG. 5 a.After the fault is cleared, the system automatically resumes normaloperation, as shown in FIG. 5 b.

Referring now to FIG. 6, a current versus time plot is shown for thebidirectional protection device 65 connected in series between a 200volt battery and a single-phase 10 kilowatt DC-to-DC converter systempowered by the 200V battery when a fault, such as a short circuit of theother power transistor in the DC-DC converter, causes the battery chargecurrent to rise sharply. In this example, the current is tripped at −168amperes and drops to about −2 amperes within 2 microseconds and thenquickly drops to near zero.

The simulated results in FIGS. 5 a, 5 b, and 6 demonstrate that thebidirectional protection device 65 operates at a very high speed and iscapable of automatically resuming normal operation once the voltage dropacross the device between the nodes 32 a and 32 b decreases to a valuethat turns OFF either of the second ECBs 7 a or 7 b after the fault iscleared.

Referring now to FIG. 7, a trip current versus temperature plot is shownfor the bidirectional protection device 65 with and without thetemperature compensation components 12 a, 12 b. The trip currentdecreases sharply with an increase in temperature without thetemperature compensation components 12 a, 12 b. The trip current is seento be much less sensitive to the temperature variation over the range of27° Celsius to 125° Celsius, when the temperature compensationcomponents 12 a, 12 b are employed.

Referring now to FIG. 8, a voltage drop versus current plot is shown toillustrate the effects of power diodes as the current bypass components18 a, 18 b on the forward voltage drop of the bidirectional protectiondevice 65 for a 10 kilowatt, 200 volt system. With the power diodes, theforward voltage drop in normal operation is reduced by about 20% or 0.3volts at the forward current up to 50 amperes. The reduced voltage dropis a direct result of the smaller ON-state voltage drop of the powerdiodes in comparison to that of MOSFETs as the first ECBs 6 a and 6 b.In general, a lower ON-state voltage drop correlates to a lowerinsertion loss.

Referring now to FIG. 9, a voltage drop versus current plot compares theforward voltage drop of the bidirectional protection devices 65 and 63for a 10 kilowatt, 200 volt system. The bidirectional protection device65 has a much smaller forward voltage drop as compared to thebidirectional protection device 63 in FIG. 3 b. The substantialreduction in the forward voltage drop or insertion loss is due to theuse of the entire voltage drop of the bidirectional protection device 65to forward bias the gate-source terminals of first ECBs 6 a, 6 b.

Referring now to FIGS. 10 a, 10 b, tripping current plots show exemplarycurrent versus time plots for the bidirectional protection device 65demonstrated experimentally for a 10 kilowatt DC-to-DC converter systempowered by a 300 volt source. FIG. 10 a shows the measured currenttripping waveforms for the surge current flowing from terminal 81 toterminal 83. FIG. 10 b shows the measured current tripping waveforms forthe surge current flowing from terminal 83 to terminal 81. Experimentalresults show that the device has a trip current very close to thedesigned target of 50 amperes and has an extremely fast turn-off speedof less than 1 microsecond.

While depletion mode circuit blocks with a negative threshold voltageand enhancement mode circuit blocks with a positive threshold voltagebased on n-channel devices are described herein, depletion mode circuitblocks with a positive threshold voltage and enhancement mode circuitblocks with a negative threshold voltage based on p-channel devices,could also be used; although, such embodiments are not preferred becauseof the large channel resistance of p-channel devices.

The voltage and current capability of the depletion mode circuit blocksand transistors described herein are chosen to meet specific protectionrequirements. For example, the protection devices are not generallyrequired to protect against a very large current, but rather protectagainst very high voltages in many telecommunication applications. Inanother example, the protection devices for electric vehicle batteriesare generally required to protect against a very large current, ratherthan very high voltages.

The description above indicates that a great degree of flexibility isoffered in terms of the invention. Although various embodiments havebeen described in considerable detail with reference to certainpreferred versions thereof, other versions are possible. Therefore, thespirit and scope of the appended claims should not be limited to thedescription of the preferred versions contained herein.

1. A protection device comprising: (a) a first current limiting load;(b) a second current limiting load; (c) a first circuit block being adepletion mode circuit with either a negative threshold voltage or apositive threshold voltage; (d) a second circuit block being a firstenhancement mode circuit; and (e) a third circuit block being a secondenhancement mode circuit, said third circuit block has a thresholdvoltage magnitude larger than said second circuit block; where saidfirst circuit block, said second circuit block, and said third circuitblock each has a drain terminal, a source terminal, and a gate terminal,said first circuit block connected in series with said second circuitblock so that said source terminal of said first circuit block isconnected to said drain terminal of said second circuit block, saiddrain terminal of said third circuit block is connected to said gateterminal of said second circuit block and further connected to saiddrain terminal of said second circuit block through said first currentlimiting load, said gate terminal of said third circuit block isconnected to said drain terminal of said second circuit block throughsaid second current limiting load, said gate terminal of said firstcircuit block is connected to said source terminals of said secondcircuit block and said third circuit block, a current path through saidfirst circuit block and said second circuit block with said thirdcircuit block controlling the ON and OFF status of said second circuitblock and said second circuit block in turn controlling the ON and OFFstatus of said first circuit block.
 2. The protection device of claim 1,wherein said first circuit block is at least one depletion moden-channel transistor, and said second circuit block and said thirdcircuit block is each at least one enhancement mode n-channeltransistor.
 3. The protection device of claim 2, further comprising: (f)a capacitor; (g) a variable resistor; (h) a temperature compensationcomponent, where said capacitor, said variable resistor and saidtemperature compensation component are connected between said gateterminal and said source terminal of said third circuit block; and (i) avoltage limiting component connected to said source terminal and saidgate terminal of each of said second circuit block and said thirdcircuit block.
 4. The protection device of claim 3, wherein said currentlimiting loads are dynamic resistors and said voltage-limitingcomponents are Zener diodes.
 5. The protection device of claim 4,wherein said dynamic resistors are each formed by a depletion modetransistor and a constant resistor, said constant resistor beingconnected between said gate terminal and said source terminals of saiddepletion mode transistor and being in the drain-to-source current pathof said depletion mode transistor.
 6. The protection device of claim 5,wherein said first circuit block is a depletion mode field-effecttransistor, a depletion mode static induction transistor, or a depletionmode IGBT, and said second circuit block and said third circuit blockare each an enhancement mode field-effect transistor, an enhancementmode static induction transistor, or an enhancement mode IGBT.
 7. Theprotection device of claim 6, further comprising: (j) a current bypassdiode connected to said source terminal and said drain terminal of saidfirst circuit block and/or said second circuit block.
 8. The protectiondevice of claim 6, further comprising: (j) an RC network connectedparallel to said protection device.
 9. A protection device comprising:(a) a first current limiting load; (b) a second current limiting load;(c) a first circuit block being a depletion mode circuit with either anegative threshold voltage or a positive threshold voltage; (d) a secondcircuit block being a first enhancement mode circuit; and (e) a thirdcircuit block being a second enhancement mode circuit, said thirdcircuit block having a threshold voltage magnitude larger than saidsecond circuit block; where said first circuit block, said secondcircuit block, said third circuit block each has a drain terminal, asource terminal, and a gate terminal, said first circuit block connectedin series with said second circuit block so that said source terminal ofsaid first circuit block is connected to said drain terminal of saidsecond circuit block, said drain terminal of said third circuit blockconnected to said gate terminal of said second circuit block and furtherconnected to said drain terminal of said first circuit block throughsaid first current limiting load, said gate terminal of said thirdcircuit block connected to said drain terminal of said first circuitblock through said second current limiting load, said gate terminal ofsaid first circuit block connected to said source terminals of saidsecond circuit block and said third circuit block, a current paththrough said first circuit block and second circuit block with saidthird circuit block controlling the ON and OFF status of said secondcircuit block and said second circuit block in turn controlling the ONand OFF status of said first circuit block.
 10. The protection device ofclaim 9, wherein said first circuit block is at least one depletion moden-channel transistor, said second circuit block is at least oneenhancement mode n-channel transistor, and said third circuit block isat least one enhancement mode n-channel transistor.
 11. The protectiondevice of claim 10, further comprising: (f) a capacitor; (g) a voltagelimiting component; (h) a variable resistor; and (i) a temperaturecompensation component; where said capacitor, said variable resistor,and said temperature compensation component are connected between saidsource terminal and said gate terminal of said third circuit block andsaid voltage limiting component is connected between said sourceterminal and said gate terminal of each of said second circuit block andsaid third circuit block.
 12. The protection device of claim 11, whereinsaid current limiting loads are dynamic resistors.
 13. The protectiondevice of claim 12, wherein said dynamic resistors are each formed by adepletion mode transistor and a constant resistor, said constantresistor being connected between said gate terminal and said sourceterminal of said depletion mode transistor and in the drain-to-sourcecurrent path of said depletion mode transistor.
 14. The protectiondevice of claim 13, wherein said first circuit block is a depletion modefield-effect transistor, a depletion mode static induction transistor,or a depletion mode IGBT, and said second circuit block and said thirdcircuit block are each an enhancement mode field-effect transistor, anenhancement mode static induction transistor, or an enhancement modeIGBT.
 15. The protection device of claim 14, further comprising: (j) acurrent bypass diode connected to said source terminal and said drainterminal of said first circuit block and/or said second circuit block.16. The protection device of claim 15, further comprising: (k) an RCnetwork connected parallel to said protection device.
 17. The protectiondevice of claim 16, wherein said first current limiting load or saidsecond current limiting load is connected at one end to said sourceterminal of said first circuit block and said drain terminal of saidsecond circuit block.
 18. A bidirectional protection device comprising:(a) a first current limiting load; (b) a second current limiting load;(c) a third current limiting load; (d) a fourth current limiting load;(e) a first circuit block being a depletion mode circuit with either anegative threshold voltage or a positive threshold voltage; (f) a secondcircuit block being an enhancement mode circuit; (g) a third circuitblock being an enhancement mode circuit; (h) a fourth circuit blockbeing a depletion mode circuit with either a negative threshold voltageor a positive threshold voltage; (i) a fifth circuit block being anenhancement mode circuit; and (j) a sixth circuit block being anenhancement mode circuit; whereby each said circuit block has a drainterminal, a source terminal, and a gate terminal, said second circuitblock, said first circuit block, said fourth circuit block, and saidfifth circuit block connected in series with said source terminal ofsaid first circuit block connected to said drain terminal of said secondcircuit block, said drain terminal of said first circuit block connectedto said drain terminal of said fourth circuit block, said sourceterminal of said forth circuit block connected to said drain terminal ofsaid fifth circuit block, said drain terminal of said third circuitblock connected to said gate terminal of said second circuit block andcoupled to said source terminal of said fifth circuit block through saidfirst current limiting load, said drain terminal of said sixth circuitblock connected to said gate terminal of said fifth circuit block andconnected to said source terminal of said second circuit block throughsaid third current limiting load, said gate terminal of said firstcircuit block and said source terminal of said third circuit block bothconnected to said source terminal of said second circuit block, saidgate terminal of said forth circuit block and said source terminal ofsaid sixth circuit block both connected to said source terminal of saidfifth circuit block, said gate terminal of said third circuit blockconnected to said source terminal of said fifth circuit block throughsaid second current limiting load, said gate terminal of said sixthcircuit block connected to said source terminal of said second circuitblock through said fourth current limiting load, a current path providedthrough said first circuit block, said second circuit block, said fourthblock, and said fifth circuit block so that said third circuit blockcontrols the ON and OFF status of said second circuit block and saidsecond circuit block in turn controls the ON and OFF status of saidfirst circuit block, and so that said sixth circuit block controls theON and OFF status of said fifth circuit block and said fifth circuitblock in turn controls the ON and OFF status of said fourth circuitblock.
 19. The bidirectional protection device of claim 18, wherein saidfirst circuit block is at least one depletion mode n-channel transistor,said second circuit block is at least one enhancement mode n-channeltransistor, said third circuit block is at least one enhancement moden-channel transistor, said fourth circuit block is at least onedepletion mode n-channel transistor, said fifth circuit block is atleast one enhancement mode n-channel transistor, and said sixth circuitblock is at least one enhancement mode n-channel transistor.
 20. Thebidirectional protection device of claim 19, further comprising: (k) acapacitor; (l) a voltage limiting component; (m) a variable resistor;and (n) a temperature compensation component; whereby said capacitor,said variable resistor, and said temperature compensation component areconnected between said source terminal and said gate terminal of each ofsaid third circuit block and said sixth block and said voltage limitingcomponent is connected between said source terminal and said gateterminal of each of said second circuit block, said third circuit block,said fifth circuit block and said sixth circuit block.
 21. Thebidirectional protection device of claim 20, wherein said currentlimiting loads are dynamic resistors.
 22. The bidirectional protectiondevice of claim 21, wherein said dynamic resistors are each formed bytwo depletion mode transistors and a constant resistor, said sourceterminal of the first one of said depletion mode transistors connectedto said constant resistor and said gate terminal of the second one ofsaid depletion mode transistors while said source terminal of the secondone of said depletion mode transistor is connected to the other terminalof said constant resistor and said gate terminal of the first one ofsaid depletion mode transistors.
 23. The bidirectional protection deviceof claim 22, wherein said first circuit block and said fourth circuitblock are each a depletion mode field-effect transistor, a depletionmode static induction transistor, or a depletion mode IGBT, and saidsecond circuit block, said third circuit block, said fifth circuitblock, and said sixth circuit block are each an enhancement modefield-effect transistor, an enhancement mode static inductiontransistor, or an enhancement mode IGBT.
 24. The bidirectionalprotection device of claim 23, further comprising: (o) a current bypassdiode connected between said source terminal and said drain terminal ofeach of said second circuit block and said fifth circuit block and/oreach of said first circuit block and said fourth circuit block.
 25. Thebidirectional protection device of claim 24, further comprising: (p) anRC network connected parallel to said bidirectional protection device.26. The bidirectional protection device of claim 24, further comprising:(p) an RC network connected parallel to said first circuit block, saidsecond circuit block and said third circuit block; and (q) an RC networkconnected parallel to said fourth circuit block, said fifth circuitblock, and said sixth circuit block.